EP0707883B1 - Catalyseur et méthode pour la purification de gaz d'échappement - Google Patents

Catalyseur et méthode pour la purification de gaz d'échappement Download PDF

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EP0707883B1
EP0707883B1 EP95307075A EP95307075A EP0707883B1 EP 0707883 B1 EP0707883 B1 EP 0707883B1 EP 95307075 A EP95307075 A EP 95307075A EP 95307075 A EP95307075 A EP 95307075A EP 0707883 B1 EP0707883 B1 EP 0707883B1
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catalyst
zone
exhaust gases
nox
loaded
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EP0707883A2 (fr
EP0707883A3 (fr
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Takashi Ito
Yukio Kosaki
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NE Chemcat Corp
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NE Chemcat Corp
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9459—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts
    • B01D53/9463—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on one brick
    • B01D53/9472—Removing one or more of nitrogen oxides, carbon monoxide, or hydrocarbons by multiple successive catalytic functions; systems with more than one different function, e.g. zone coated catalysts with catalysts positioned on one brick in different zones
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9404—Removing only nitrogen compounds
    • B01D53/9409—Nitrogen oxides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/944—Simultaneously removing carbon monoxide, hydrocarbons or carbon making use of oxidation catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1021—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1025—Rhodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1028—Iridium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/104—Silver
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
    • Y02C20/00—Capture or disposal of greenhouse gases
    • Y02C20/10—Capture or disposal of greenhouse gases of nitrous oxide (N2O)

Definitions

  • the present invention relates to a two-step catalyst system and method using that catalyst system for purifying exhaust gases which are exhausted from internal combustion engines, such as a lean-burn engine, a 2-stroke engine and a diesel engine which are driven at higher ratio of air to fuel (lean-burn air to fuel ratio), and from boilers, gas turbines and so on.
  • exhaust gases typically contain excess oxygen and nitrogen oxides.
  • Certain catalysts which cause direct catalytic decomposition of nitrogen oxides are known.
  • Cu ion-exchanged ZSM-5 Japanese Patent Laid-Open No. Shou 60-125250 (85/125250)
  • certain perovskite catalysts containing Cu or Co Teoka et al., Chem. Lett., 1 (1990); Shimada et al., Chem, Lett., 1797 (1988)
  • certain Pd/MgO catalyst Ogata et al., Appl, Catal. , 65 :L11 (1990)
  • the temperature range of effective NOx purification is relatively high at higher than 350°C, and in exhaust gases containing water at higher than 650°C serious thermal deterioration of the catalyst can occur.
  • a catalyst such as the above mentioned (2), at relatively low temperatures of 200°C to 300°C NOx is reduced, but the effective temperature range is narrow and a considerable amount of N 2 O can be produced as partial reduction product of NOx. Accordingly, these catalysts are generally not suitable for practical use.
  • the above mentioned NOx decomposition catalyst 1) is said to have an activity at 300°C to 500°C, but in the presence of water (H 2 O) and oxygen (O 2 ) the catalyst can become deactivated.
  • the above mentioned catalysts 2) and 3) also have problems such as that the effective temperature range of 700°C to 800°C is too high, rendering the catalysts generally not suitable for practical use.
  • the above mentioned systems may not be capable of meeting more stringent exhaust gas control standards than current standards, including such standards that may be effective in near future (e.g. HC control value of ULEV in California, USA).
  • the catalysts mentioned in (3) and (4) above have a problem of producing N 2 O as a by-product.
  • the present invention has overcome such problems of the prior art.
  • the present invention provides a novel and effective catalyst system and method for purifying of exhaust gases containing a reductive component comprising hydrocarbon, nitrogen oxides and oxygen in excess of a stoichiometrical amount.
  • the catalyst system and method of the invention achieves a high level of denitrification at a wide range of temperature, including from 200°C to 500°C, can achieve high levels of purification of HC and CO at more than 200°C, and produces no or at least substantially no N 2 O as a by-product.
  • Figure 1 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Comparative Sample 1.
  • Figure 2 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Comparative Sample 2.
  • Figure 3 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Comparative Sample 3.
  • Figure 4 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Example 1 of the present invention.
  • Figure 5 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Comparative Sample 4.
  • Figure 6 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Example 2 of the present invention.
  • Figure 7 is a graph showing the purifying ability of exhaust model gas of lean-burn engines as obtained in Example 3 of the present invention.
  • the methods of the invention include purifying exhaust gases which comprise a reduction component containing hydrocarbon and greater amount of oxygen and nitrogen oxides than the stoichiometrical amount needing to perfectly oxidize all the reduction component.
  • the methods include placing at least two catalyst zones along side, or in the flow path, of the exhaust gas stream.
  • the first catalyst zone i.e. the upstream catalyst area
  • the second catalyst zone downstream from the first zone (i.e.
  • the present inventors have studied the selective reduction reaction of NOx by HC in lean-burn engine exhaust model gases in the presence of platinum catalyst on carrier. It has been found that a certain platinum catalyst on a carrier provide the higher conversion of NOx under the lower concentration of HC, contrary to prior reports. It was also found that the highest conversion of NOx was achieved in the absence of HC (i.e. zero or essentially zero HC concentration), and that N 2 O was not produced as a by-product in such absence of HC. According to these findings, methods to purify exhaust gases containing high ratios of oxygen and which achieve high levels of removal of CO and HC, high levels of removal of NOx and produce low levels of N 2 O have been provided, and the present invention has been accomplished.
  • perfect oxidation reaction of HC means a reaction producing only CO 2 and H 2 O from HC and O 2 without producing alcohol, aldehydes, ketone, CO, etc. Such a perfect oxidation reaction is distinct from a partial oxidation reaction which produces alcohol, aldehyde, ketone, CO and so on.
  • the catalyst system for purifying exhaust gases of the present invention is a catalyst system for purifying exhaust gases containing a reductive component comprising hydrocarbon, nitrogen oxides and oxygen in excess of the stoichiometrical amount to perfectly oxidize all the reductive component.
  • the catalyst system includes locating a hydrocarbon oxidation catalyst having high perfect oxidation activity of hydrocarbon at a first zone in the exhaust gases' flow path, and a NOx decomposition catalyst that can decompose nitrogen oxides to nitrogens and oxygens at a second zone in the exhaust gases, downstream in the exhaust gases' flow path from the first zone.
  • the catalyst in the first zone can achieve perfect oxidation for hydrocarbons to CO 2 and H 2 O, more preferably can achieve high or perfect oxidation for hydrocarbon to CO 2 and H 2 O.
  • the conversion HC in the first catalyst zone is preferably about 50% or more, more preferably about 90% or more (expressed as volume volume %; gas compositions expressed herein as volume % unless otherwise indicated). If the first zone catalyst does not have high perfect oxidation ability for hydrocarbon, a considerable amount of HC is not oxidized and such HC leaves the first zone catalyst, or CO, a partial oxidization product of HC, is produced. Such remaining reductive compound can negatively impact the activity and selectivity of the NOx decomposition catalyst located at the second zone downstream in the exhaust gases' flow path from the first zone.
  • CO in exhaust gases is an intermediate produced by partial oxidation of HC.
  • a catalyst having high perfect oxidation ability for HC necessary promotes the oxidation of CO to CO 2 .
  • the catalyst having high oxidation activity of HC necessarily has high oxidation activity of CO in the exhaust gases that contain an excess amount of oxygen.
  • Such HC oxidation catalyst at the first catalyst zone is preferable to have lower convertive (oxidation/reduction) activity of NO. In the presence of excess amount of oxygen, NO reacts with O 2 and is oxidized to NO 2 , or NO is partially reduced by HC and N 2 O is produced.
  • the first zone catalyst preferably does not cause NO to be oxidized to NO 2 , and does not cause NO to be reduced to N 2 O by the HC oxidation catalyst to the largest extent possible.
  • the catalyst preferably has superior low temperature activity.
  • the first zone catalyst near the exhaust manifold to improve the removal ratio of CO and HC to a high level as may be required under future emission standards as discussed above.
  • the first zone catalyst preferably has high temperature thermal resistance, including thermal resistance (i.e. does not decompose during regular use) up to about 900°C.
  • the first zone catalysts are those which comprise at least one metal selected from the group consisting of platinum, palladium, rhodium and iridium in the presence of gold loaded on a refractory porous metal oxide carrier, particularly to achieve high perfect oxidation ability of HC and to control NO conversion activity.
  • Exemplary carriers of such loaded noble metal catalyst include refractory inorganic carriers, for example alumina, silica, titania, zirconia, silica-alumina, titania-silica, aluminosilicate, aluminophosphate, silicoaluminophosphate, zirconiumphosphate, siliconcarbide, etc.; and more preferable are the refractory porous metal oxide carriers, such as activated alumina, silica, silica-alumina.
  • refractory inorganic carriers for example alumina, silica, titania, zirconia, silica-alumina, titania-silica, aluminosilicate, aluminophosphate, silicoaluminophosphate, zirconiumphosphate, siliconcarbide, etc.
  • refractory porous metal oxide carriers such as activated alumina, silica, silica-alumina.
  • the amount of the noble element loaded on the first zone catalyst can suitably vary, and is generally about 0.1% to 5.0% by weight to gross weight of the catalyst, preferably about 0.5% to 2.0% by weight.
  • total loaded amount of the noble metal is 0.1% to 5.0% by weight, and is preferably 0.5% to 2.0% by weight. If the loaded amount is too small, HC oxidation activity may not be sufficient. If the loaded amount is too large, the dispersion degree of the noble metal may decrease. In these cases, the light-off temperature of HC oxidation reaction increases, HC conversion ratio decreases and the extent of the NO conversion reaction tends to increases, which is less desirable.
  • the ratio of loaded amount of elements of the platinum group to gold is suitably about 5:1 to 100:1 by weight. If the loaded amount of gold is too large, HC oxidation activity can be prevented or inhibited, and if the loaded amount of gold is too small, control of NO conversion activity may be prevented or inhibited.
  • the catalysts may be shaped as desired, or the catalysts may be coated and used on pre-shaped refractory support substrate.
  • a first zone catalyst may be constructed with a housing into which the HC catalyst of any shape can be charged or loaded.
  • the shape of the first catalyst is not limited and for example beads, pellets, rings, honeycomb and/or foam can be used.
  • Preferred is a first zone catalyst which comprises the catalyst itself coated onto a honeycomb-shaped monolith support substrate having many open channels along the exhaust gases' flow and formed from refractory ceramics, such as cordierite, mullite, siliconcarbide, etc., stainless steels, etc.
  • Such a catalyst structure can increase contact area of the catalyst and exhaust gases with control of pressure drop.
  • the second zone catalyst of the method for purifying exhaust gases of the present invention which the exhaust gases connect after CO and HC of the gases have been oxidized upstream in the exhaust gases' flow on the first zone catalyst, is not particularly limited, except that the catalyst should promote catalytic NOx decomposition under a lowered HC concentration or in the substantial absence of HC and in the presence of excess oxygen.
  • the NOx purification ratio (defined below) is suitably at least about 20%, preferably 30% or more, more preferably 40% or more.
  • Exemplary of such second zone NOx decomposition catalyst in the presence of excess oxygen include catalysts which comprise at least one metal selected from group consisting of platinum, rhodium, iridium, preferably loaded on a refractory inorganic support.
  • the NOx decomposition activity may be raised by the carrier used and the manner in which the carrier is loaded with active metal.
  • a second zone catalyst comprising platinum loaded on porous metal oxide carriers such as alumina, silica, titania, zirconia, silica-alumina etc.
  • the loaded amount of noble metal element of the second area catalyst is generally about 0.5% to 15% by weight to gross weight of the catalyst, preferably, about 3% to 10% by weight. If the loaded amount of noble metal is too small, NOx decomposition activity may not be sufficient, and if the loaded amount of noble metal is too large, NOx decomposition activity may not increase in proportion to increase of the loaded amount, which is less preferred.
  • platinum loaded on active alumina carrier at the ratio of about 3% to 10% by weight is especially preferable.
  • the amount of HC with respect to NOx is high in the exhaust gases that contact the second zone loaded noble catalyst, especially loaded platinum catalyst, the reduction of NOx by HC can occur and the production of N 2 O can increase.
  • the NOx decomposition reaction takes precedence over the reduction reaction of NOx by HC.
  • the upper limit of the ratio of the HC concentration (methane reduced concentration of HC, referred to herein as THC) to NOx concentration (THC/NOx) in the exhaust gases so that the NOx decomposition reaction is promoted may be determined depending on the types of HC in the exhaust gases or on reaction conditions.
  • the ratio of THC to NOx is less than 1, more preferably the ratio THC/NOx is less than 0.3.
  • the method of the present invention is not limited to the above range of the ratio of THC to NOx, particularly if only exhaust gases, having lower HC concentration than the HC concentration of the exhaust gases at the inlet (upstream portion) of the first zone catalyst, contact the second zone catalyst so that NOx decomposition activity of the second catalyst area may be enhanced and the effect of the method of the present invention can be achieved.
  • the catalyst of the second zone may be formed itself in a certain shape, or the catalyst may be coated onto and used on a pre-shaped refractory support substrate.
  • a second zone catalyst may be constructed with a housing into which the NOx decomposition catalyst of any shape can be charged or loaded.
  • the shape of the second zone catalyst is not limited and for example beads, pellets, rings, honeycomb and/or foam can be used.
  • Preferred is a second zone catalyst which comprises the catalyst itself coated onto a honeycomb-shaped monolith support substrate having many open channels to permit the exhaust gases' flow therethrough and formed refractory ceramics such as cordierite, mullite, siliconcarbide etc., stainless steels, etc.
  • Such a catalyst structure can increase contact area of the catalyst and exhaust gases with control of pressure drop.
  • first and second zone catalysts can be coated onto the same brick or unit of honeycomb substrate with separation of the respective zones of application of the first and second zone catalysts so that the first zone catalyst is coated onto an upstream area or zone of the honeycomb structure upon placement of the structure in an exhaust gas flowpath, and the second zone catalyst is coated onto a downstream area of the same honeycomb structure upon placement of the structure in an exhaust gas flowpath.
  • the method of the present invention for purifying the exhaust gases containing reductive component comprising hydrocarbon, nitrogen oxides and excess amount of oxygen than the stoichiometrical amount to perfectly oxidize all the reductive component is the method in which said exhaust gases contact with HC oxidation catalyst having higher perfect oxidation ability of HC at the first zone in the gas flow, and secondly the gases contact with NOx decomposition catalyst which decomposes NOx to N 2 in the second zone downstream in the exhaust gases flowpath with respect to the first zone catalyst and in the presence of a lower concentration of HC relative to the concentration upstream at the first zone catalyst, or in the substantial absence of HC.
  • the purifying method of the present invention CO and HC in the exhaust gases containing excess oxygen are perfectly oxidized at higher conversion and removed and NOx is decomposed and removed with no or essentially no production of N 2 O as a by-product.
  • Use of the catalysts system of the invention preferably provides a N 2 O production ratio (defined below) of 20% or less, more preferably about 10% or less, still more preferably a N 2 O production ratio of about 7% or less.
  • the inlet gas temperature of the first zone and second zone catalyst (i.e. the temperature of the exhaust gas as it contacts the first and second zone catalysts respectively) has particular no limitation, except the inlet gas temperature should be sufficient to perfectly oxidize HC and CO, control NOx conversion reaction for the first zone catalyst and effectively decompose NOx (i.e. essentially no production of N 2 O as a by-product) for the second zone catalyst, respectively.
  • the inlet gas temperature of the first zone catalyst is higher than 200°C and lower than 700°C. If the temperature is too low, HC perfect oxidation activity make not be sufficient, HC concentration in exhaust gases supplied to the second zone catalyst may increase, and the selectivity of NOx decomposition reaction in the second zone catalyst may decrease. If the inlet gas temperature of the first zone catalyst is too high, aggregation (sintering) of the catalytic active component can occur and the higher activity of HC oxidation may be inhibited.
  • the inlet gas temperature of the second zone catalyst is preferably from about 150°C to 450°C, more preferably the temperature is from about 200°C to 400°C. If the inlet temperature is too low, NOx decomposition reaction may be inhibited, and if it is too high, NOx removal ratio may decrease. Generally, other than any exothermal reaction heat generated on the first catalyst zone, the further exhaust gases have flowed from a gas generating source the cooler the exhaust gases will be, and the inlet gas temperature of the second zone catalyst can be made lower than the inlet gas temperature of the first zone catalyst for the same exhaust gas.
  • the second zone catalyst may be placed directly behind the first zone catalyst or may be placed at a certain distance from the first zone catalyst. The positioning and attachment of the first and second zone catalysts in the exhaust gas flow path may be selected to achieve the most effective result.
  • both the first and second catalyst zones can be positioned in a single housing placed in the exhaust gas flow path.
  • two or more housings can be placed in the exhaust gas flow path, a first upstream housing containing the first catalyst zone and a second downstream housing containing the second catalyst zone.
  • gas space velocity of exhaust gases which contact with the first zone catalyst and the second zone catalyst have no particular limitation, and generally range from about 5,000/hr to 200,000/hr.
  • gas space velocity of exhaust gases which contact with the first zone catalyst is from about 30,000/hr to 150,000/hr
  • the gas space velocity of the second zone catalyst preferably is from about 10,000/hr to 100,000/hr. If the gas space velocity is too small, a larger capacity of catalyst may be needed, which is less desirable. If the gas space velocity is too large, the HC light-off temperature of the first zone catalyst shifts to the higher temperature side, the HC conversion ratio may decrease, and the NOx conversion ratio of the second zone catalyst may also decrease, which is less desirable.
  • the ratio of space velocity of the first zone catalyst to that of the second zone catalyst also is not particularly limited, and preferably SV 1 /SV 2 is less than 10 and more than 0.1, and more preferably SV 1 /SV 2 is less than 3 and more than 0.5.
  • NOx conversion ratio A - B A x 100%
  • N 2 O producing ratio C x 2 A x 100%
  • A, B and C of the above formulae are defined to mean as follows:
  • the oxidation catalyst of the first zone is perfect oxidation catalyst of HC, it can increase the purification ratio of CO and HC to the same level as or higher level than the prior three-way catalyst, and it can be easier to increase heat resistance at high temperature.
  • the NOx decomposition catalyst of the second zone displays high NOx decomposition ratio under the condition that concentration of reductant of HC etc. is at a sufficiently low level such as exhaust gases after passing the first zone catalyst, or under in the substantial absence of HC.
  • the method of purifying of the present invention is extremely effective method in respect of utilizing combination of advantages of two types of catalyst which promote different types of reactions respectively, and the method of the invention has great utility to purify exhaust gases of lean-burn engines.
  • the honeycomb coated with Pt-Au loaded alumina catalyst (1) was packed in stainless steel reaction tube having internal diameter of 30 mm with winding glass wool on the side of the honeycomb core for preventing gas blowing.
  • a mixed gas (model gas G-1) containing 1,000 ppm (all gas compositions expressed herein as volume % unless otherwise indicated) of NO, 1,500 ppm of propylene as HC, 1,000 ppm of CO, 7.5% of O 2 , 10% of CO 2 , 10% of H 2 O and the balance being N 2 was flowed into the reaction tube at a flow rate of 16.1 L/min (SV 30,000/hr).
  • the inlet gas temperature of the catalyst layer was successively raised from 100°C to 500°C at a rate of 30°C/minute.
  • the concentration of NOx, CO, HC and N 2 O of the gases exiting from the catalyst layer were measured by a NOx measuring instrument, a CO measuring instrument, a HC measuring instrument and a N 2 O measuring instrument respectively.
  • the conversion ratio of NOx, CO and HC and the production ratio of N 2 O with respect to the inlet gas temperatures of the catalyst layer (light-off performance) was assessed.
  • the light-off performance of NOx, CO, HC and N 2 O on the assessment of the model gas of Comparative Sample 1 is shown in Figure 1 of the Drawings.
  • the catalyst (1) showed high conversion ratio of CO, HC of more than 95% at inlet temperatures of higher than 280°C; however, the catalyst showed less than 7% of the conversion ratio of NOx at temperature ranges of from 100°C to 500°C.
  • the results showed that the catalyst (1) had high perfect oxidation ability of CO and HC, but caused almost no reduction of NOx by HC. Hence, catalyst (1) hardly produced N 2 O, with less than 6% of the producing ratio of N 2 O.
  • the results are shown in Figure 2 of the Drawings.
  • the results show the catalyst (2) was a catalyst that exhibited high conversion ratio of CO, HC of more than 98% at inlet temperature of the catalyst of higher than 220°C to 500°C, but showed less than 11% of the conversion ratio of NOx at a range of temperature from 100°C to 500°C, and did not convert NOx but effectively oxidized CO and HC, and hardly produced N 2 O. Also, other assessments through use of a NO measuring instrument showed that the catalyst (2) hardly oxidized NO to NO 2 .
  • the results are shown in Figure 3 of the Drawings.
  • the catalyst (3) showed high conversion ratio of CO, HC of more than 95% at entrance temperatures of higher than 250°C, and achieved NOx converting reaction at the same time, and achieved a maximum NOx conversion ratio of 50% at nearly 240°C.
  • the catalyst (3) achieved N 2 O production ratio of 38% at maximum, converted nearly 70% to 80% of converted NOx to N 2 O, and showed remarkable low NOx purification ratio at temperatures greater than 350°C.
  • the result showed that a mere loaded noble metal catalyst, such as the prior TWCs (three way catalysts), caused partial reduction of NO to N 2 O by HC at the same time as oxidation of CO and HC, and had an imperfect purifying ability of NOx.
  • the catalyst (4) showed a preferable conversion ratio of NOx of more than 40% at wide range of temperature from 280°C to 400°C, generated hardly N 2 O, and effectively made NOx harmless.
  • the model gas G-2 is a model gas system with CO and HC removed from the model gas G-1 as described above by the perfect oxidation, and can be regarded as an outlet gas of a catalyst after the model gas G-1 has contacted HC oxidation catalyst (1) or (2). Therefore, it is clear that the NOx decomposition catalyst of the present invention showed superior denitrification ability for such exhaust gas containing no HC.
  • the catalyst (4) which showed superior purifying ability of NOx in a non-HC system in Example 1 above, produced N 2 O from nearly 80% of converted NOx in the presence of considerable amounts of HC to NOx. It was clear that the catalyst (4), which predominately caused NOx decomposition reaction in a non-HC system, predominately caused a reduction reaction of NOx by HC at HC existing system rather than decomposition reaction of NOx, and on such reduction reaction of NOx by HC, NOx was not rendered harmless. Hence, it was clear that N 2 O production in the latter system is not controlled in the method of treating exhaust gases described in the above mentioned reports (Japanese Patent Laid-Open No.
  • the catalysts showed high conversion ratio of CO, HC of more than 95% at higher than 280°C and also good NOx conversion ratio of more than 30% at relatively wide range of temperature from 250°C to 400°C.
  • the catalysts also showed a N 2 O production ratio of less than 5% at all temperatures examined in the range of 100°C to 500°C.
  • placement of the HC oxidation catalyst upstream in the exhaust gases provides the ability to purify NOx.
  • use of a NOx decomposition catalyst downstream from the first zone catalyst provides the ability to effectively purify NOx.
  • the ability of a catalyst system of the invention to purify the exhaust model gas G-1 was determined in the same manner as described in Comparative Sample 5 above, except the honeycomb structure coated with the catalyst (2) was used as the first zone upstream catalyst in place of the honeycomb coated with the catalyst (1) employed in Comparative Sample 5.
  • the effect of the inlet gas temperature of the upstream catalyst layer on the conversion ratio of NOx, CO, HC and N 2 O production ratio is shown in Figure 7.
  • the catalyst system showed high conversion ratio of CO, HC of more than 97% at higher than 250°C and also good NOx conversion ratio of more than 30% at wide range of temperature from 220°C to 380°C.
  • the catalysts also showed N 2 O producing ratio of less than 5% throughout the temperature range of from 100°C to 500°C.
  • N 2 O producing ratio of less than 5% throughout the temperature range of from 100°C to 500°C.

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Claims (6)

  1. Système de catalyseurs à deux étapes pour purifier des gaz d'échappement, les gaz contenant un composant de réduction comprenant un hydrocarbure, des oxydes d'azote et de l'oxygène en excès de la quantité stoechiométrique requise pour oxyder parfaitement tout le composant de réduction,
       le système de catalyseurs comprenant un catalyseur d'oxydation d'hydrocarbures ayant une activité d'oxydation des hydrocarbures au niveau d'une première zone dans la voie d'écoulement des gaz d'échappement, et un catalyseur de décomposition de NOx pour décomposer les oxydes d'azote en azote et en oxygène au niveau d'une deuxième zone dans la voie d'écoulement des gaz d'échappement en aval de la première zone,
       dans lequel le catalyseur de première zone comprend de l'or et au moins une deuxième matière sélectionnée à partir du platine, palladium, rhodium et iridium, chargés sur un porteur réfractaire poreux.
  2. Système de catalyseurs à deux étapes selon la revendication 1 dans lequel le catalyseur de deuxième zone est du platine chargé sur un porteur réfractaire poreux.
  3. Méthode de purification des gaz d'échappement, Les gaz comprenant un composant de réduction comprenant un hydrocarbure, des oxydes d'azote et de l'oxygène en excès de la quantité stoechiométrique requise pour oxyder parfaitement tout le composant de réduction,
       la méthode comprenant mettre en contact les gaz d'échappement avec le catalyseur d'oxydation d'hydrocarbures ayant une activité d'oxydation d'hydrocarbures au niveau de la première zone de la voie d'écoulement des gaz d'échappement, et ensuite mettre en contact les gaz d'échappement avec un catalyseur de décomposition de NOx pour décomposer les oxydes d'azote en azote et en oxygène au niveau d'une deuxième zone de la voie d'écoulement en aval de la première zone,
       dans laquelle le catalyseur de première zone comprend de l'or et au moins une deuxième matière sélectionnée à partir du platine, palladium, rhodium et iridium, chargés sur un porteur réfractaire poreux.
  4. Méthode selon la revendication 3 dans laquelle la concentration d'hydrocarbure dans les gaz d'échappement au niveau de l'admission du catalyseur de la deuxième zone est inférieure à celle au niveau de l'admission du catalyseur de la première zone.
  5. Méthode selon la revendication 3 ou 4 dans laquelle le catalyseur de deuxième zone est du platine chargé sur un porteur réfractaire poreux.
  6. Méthode selon l'une quelconque des revendications 3 à 5 dans laquelle le taux de purification de NOx est de 20% ou plus et le taux de production de N2O est de 20% ou moins.
EP95307075A 1994-10-06 1995-10-05 Catalyseur et méthode pour la purification de gaz d'échappement Expired - Lifetime EP0707883B1 (fr)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
JP27820994 1994-10-06
JP6278209A JPH08103656A (ja) 1994-10-06 1994-10-06 排気ガスの浄化用触媒及びその方法
JP278209/94 1994-10-06

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EP0707883A2 EP0707883A2 (fr) 1996-04-24
EP0707883A3 EP0707883A3 (fr) 1996-09-25
EP0707883B1 true EP0707883B1 (fr) 2000-05-31

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JP (1) JPH08103656A (fr)
AT (1) ATE193461T1 (fr)
CA (1) CA2159956A1 (fr)
DE (1) DE69517279T2 (fr)

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GB0308944D0 (en) * 2003-04-17 2003-05-28 Johnson Matthey Plc Method of decomposing nitrogen dioxide
MXPA05011129A (es) 2003-04-17 2005-12-14 Johnson Matthey Plc Metodo para descomponer el dioxido de nitrogeno.
GB0422549D0 (en) 2004-10-12 2004-11-10 Johnson Matthey Plc Method of decomposing nitrogen dioxide
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JP5639337B2 (ja) * 2006-03-30 2014-12-10 ユミコア日本触媒株式会社 内燃機関排気ガスの浄化方法
US20080125313A1 (en) * 2006-11-27 2008-05-29 Fujdala Kyle L Engine Exhaust Catalysts Containing Palladium-Gold
US7534738B2 (en) 2006-11-27 2009-05-19 Nanostellar, Inc. Engine exhaust catalysts containing palladium-gold
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KR101051874B1 (ko) 2007-01-17 2011-07-25 나노스텔라 인코포레이티드 팔라듐-금을 함유하는 엔진 배기가스 촉매
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Also Published As

Publication number Publication date
DE69517279T2 (de) 2001-02-22
JPH08103656A (ja) 1996-04-23
EP0707883A2 (fr) 1996-04-24
DE69517279D1 (de) 2000-07-06
ATE193461T1 (de) 2000-06-15
CA2159956A1 (fr) 1996-04-07
EP0707883A3 (fr) 1996-09-25

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